气侵初期环空压力瞬态变化规律研究

Transient Variation of Annular Pressure at the Initial Stage of Gas Influx

文章在考虑了气侵初期水锤效应对井底压力影响的基础上,建立了水锤—漂移流综合模型来准确描述气侵过程中环空压力的瞬态变化规律,并采用复合差分格式对该模型进行求解。结果显示气侵发生初期,地层气体迅速进入井筒会引发气液交界面以上钻井液柱产生水锤效应,导致环空压力升高并随时间周期性波动,波动振幅随瞬态气侵速率的增加和瞬态气侵时间的减少而增大,而波动频率仅随瞬态气侵时间的减少而增大;在气液交界面以下的两相流区域内环空压力随时间近似线性降低。此外,地层渗透率、井底负压差越大,井眼尺寸越小,则瞬态气侵速率越大,水锤效应越显著,两相流区域内的环空压力下降速度越快。气侵初期的井底压力变化为早期气侵检测提供了可靠的理论依据。

On the basis of considering the influence of water hammer effect on bottomhole pressure at the initial stage of gas influx,an integrated model of water hammer and drift flow is established to accurately describe the transient variation law of annular pressure during gas influx,and the composite difference scheme is used to solve the model.Calculation results show that,at the early stage of gas influx,a water hammer phenomenon of mud column above the gas-liquid interface is generated,which results in the annular pressure increasing and periodically changing with time.The changing amplitude of annular pressure increases with the increase of the transient gas influx rate and the decrease of the transient gas influx time,while the fluctuation frequency increases only with the decrease of the transient gas influx time.The annular pressure below the gas-liquid interface decreases approximately linearly with time.In addition,the larger the formation permeability and negative pressure difference are and the smaller the borehole size is,the larger the transient gas influx rate is,the more significant the water hammer effect is,and the faster the annular pressure drops in two-phase flow region.The bottom hole pressure at the initial stage of gas influx can provide a reliable theoretical basis for early gas influx detection.